now querying is possible without specifying a value for each intermediate category

the perceived feature is grouping by dimension category values BEFORE grouping by date

In this example:

(def res (api/get-timeseries dosierer.server/ts
                             :registrations
                             :plants
                             {:department nil
                              :plants nil}
                              #inst "1977-01-01" #inst "2020-01-01"
                              :month))

(keys (fill-range #inst "1977-01-01" #inst "2020-01-01"
                   :month
                   (collapse res :month)))

Keys are grouping dimensions:
({:plants "4"} {:plants "5"} {:plants "6"} {:plants "2"} {:plants "1"} {:plants "3"})